Messaging systems such as the short messaging service (SMS) and the like are extremely popular types of text (or multimedia in the case of EMS and MMS) based communication provided by mobile telephone systems. However, by their very nature, their use is limited to their intended purpose.
The SMS communications protocol and implementation was designed to allow limited length alpha-numeric messages to be asynchronously transmitted relatively quickly between mobile telephones.
FIG. 1 illustrates a standard implementation of an SMS system. A Short Message Center (SMSC) stores and forwards messages to and from a mobile telephone (MS). A Short Message Entity (SME), which can be located in the fixed network or a mobile telephone, receives and sends SMS messages. A Mobile Switching Center (MSC) switches connections in a mobile telephone network between mobile telephones or between mobile telephones and the fixed network. The SMS gateway MSC (GWMS) is a gateway MSC that can also receive SMS messages. The gateway MSC is a mobile network's point of contact with other networks. On receiving the short message from the short message center, the GMSC typically uses an SS7 network to interrogate the current position of the mobile telephone from the home location register (HLR). The HLR is the main database in a mobile network. It holds information of the subscription profile of the mobile telephone and also about the routing information for the subscriber, i.e. the area (covered by a MSC) where the mobile telephone is currently situated. The GMSC is thus able to pass on the message to the correct MSC. A Visitor Location Register (VLR) corresponds to each MSC and contains temporary information about the mobile telephone, information like mobile telephone identification and the cell (or a group of cells) where the mobile telephone is currently situated. Using information form the VLR the MSC is able to switch the information (short message) to the corresponding Base Station System (BSS), which transmits the short message to the mobile telephone. The BSS consists of transceivers, which send and receive information over the air interface, to and from the mobile telephone. This information is passed over the signaling channels so the mobile telephone can receive messages even if a voice or data call is going on.
The SMS protocol used for encoding and transmission of SMS messages is described by GSM document 03.40. Whilst SMS messages are an extremely popular mechanism used for communication over mobile telephone networks, their use is limited by the implementation offered by the mobile telephone network provider. To date, most SMS systems offer basic text based communication for a fixed fee (dependent on the contract negotiated with the mobile telephone network provider).